B-WP5 - ATMOS-MODINI

The coupled atmosphere/ocean/sea-ice models used for prediction on seasonal and decadal time scales typically exhibit a cold bias in North Atlantic sea surface temperature (SST) due to the misplacement of the Gulf Stream and the North Atlantic Current (see Figure 1). The cold bias is known to impact the overlying atmospheric circulation, leading to a bias in the atmospheric circulation over the North Atlantic as well as affecting the atmospheric variability in the Euro-Atlantic sector, including Germany.

Figure 1: A schematic showing the typical path of the Gulf Stream and North Atlantic Current in coupled forecast models. The contours follow the observed surface circulation. A large discrepancy is seen in the latitude band between Canada and Europe. This is the region occupied by the North Atlantic cold bias.

The first aim of ATMOS-MODINI is to alleviate the cold bias in the MiKlip prediction system by introducing a correction to the path of the Gulf Stream and the North Atlantic Current in the model and then to test the impact of this correction on the variability and forecast skill of the model, the latter by carrying out retrospective forecasts (also known as hindcasts).

The second aim of ATMOS-MODINI is to explore ways to improve the initialisation of the MiKlip system in the tropics. It is well known that the tropics are a source of predictability for the extratropical atmosphere. However, the initialisation of a forecast system in the tropics is more difficult than at other latitudes because of the vanishing at the equator of the Coriolis force due to the Earth’s rotation.

In the first phase of MiKlip we explored the MODINI initialisation technique – a simple technique that uses the time series of observed wind stress applied to the ocean model component to initialise a coupled forecast model. We showed that hindcasts initialised using MODINI have skill in the Pacific sector out to decadal time scales with implications for forecasting global mean surface air temperature as well as the atmospheric circulation.

Figure 2 shows a forecast initialised on January 1, 2015, using MODINI indicating warmer temperatures in the tropical Pacific that persist at least to 2024. The forecast also accurately predicted that 2015, globally, would be the warmest year ever recorded. ATMOS-MODINI will continue to the explore MODINI initialisation as well as ways to combine the MODINI initialisation with other initialisation procedures used in MiKlip.

Figure 2: Forecasts for surface air temperature in Celsius compared to the 1990-2006 mean. Red indicates a forecast for warmer temperatures than for the period 1990-2006, blue colder temperatures (see Thoma et al., 2015, for the details).

Progress so far

With respect to the first aim, a high resolution ocean model (NEMO at 1/20 degree resolution, VIKING20), that simulates a realistic path of the North Atlantic Current, has been analysed (Wang et al., submitted). The potential energy term (associated with the JEBAR term) is found to play a major role, pointing to the importance of the interaction between the deep circulation and the variable bottom topography in the dynamics of the North Atlantic Current. Furthermore, an implementation of the flow field correction in the MPI-ESM-LR, based on Drews et al. (2015), has been successfully carried out. The North Atlantic cold bias has been largely removed in a pre-industrial control simulation, with no need for surface freshwater flux correction.

Concerning the second aim, a suite of hindcasts has been completed, initialized from an initialization run using reanalysis wind stress anomalies added to the model climatological wind stress (MODINI), similar to Thoma et al. (2015). Here, the ERA-40 reanalysis has been used for the period 1958-1989 (ERA-Interim for 1990-2016). The hindcasts have been carried out as part of the joint effort within MiKlip II for the intercomparison of initialisation methods.